An endoscope is an optical instrument for viewing and examining the interior of various body cavities, such as the lung, bladder, abdominal cabity or knee joint. Access to the interior body region may be via a natural body conduit or, in the case of body cavities not so accessible, by a small surgical incision. Broadly, the endoscope comprises a long thin tube, the distal (leading) end of which is inserted into the patient. An eyepiece is mounted to the proximal end of the tube, and within the tube are housed an image-forming objective at the distal end, fiber optic or discrete transfer optical elements for transmitting the optical image formed by the objective to the eyepiece outside the body for viewing by the examining physician, and fiber optic elements for transmitting light from outside the patient to the interior regions under examination for illumination thereof. The illumination fiber optics typically occupy an annular region surrounding the objective and transfer optical elements. When discrete transfer optics are used rather than fiber optics, the transfer optics typically include a front field lens which defines the image plane for all magnification conditions, and alternating relay and field lenses. The endoscope may be rigid or flexible, depending on its intended use.
The realm of endoscopic procedures includes, in addition to examination, the excision and removal of tissue. For example, it is a common technique to remove polyps and tumors by such techniques, thus avoiding open surgery. This is accomplished by providing blades or hot wires at the end of the endoscope for excision, and appropriate conduits for withdrawal of the excised tissue.
It often happens that endoscopic exploration is performed in conjunction with a biopsy. In such a procedure, the surgeon explores the interior of the body cavity, and upon noticing a suspicious looking region having a tumor, excises a small piece of tissue which is withdrawn for the biopsy. The biopsy is a procedure involving sectioning and microscopic examination in a pathology lab. The result of the biopsy is typically made available within a day or two, and if the results indicate a malignancy, the patient submits to surgery for removal of the tumor, or undergoes other appropriate treatment.
It is immediately apparent that while the use of the endoscope eliminates open surgery for the performing of the biopsy, and further can even avoid open surgery for removal of the tumor, the present procedure involving an intermediate pathological examination requires that the patient be subjected to two surgical procedures, each of which may have to be accompanied by a general anesthetic. Furthermore, the biopsy results may cause the surgeon to wish to perform further biopsy procedures.
This difficulty could be ovecome if an endoscope capable of viewing microscopically as well as telescopically could be used. With such an instrument, the examining physician could scan the interior region, and upon noticing a suspicious region, directly view in situ the single cells to make a pathological determination during the course of a single endoscopic procedure. Variable magnification endoscopes re known. A typical technique for achieving this result is shown in U.S. Pat. Nos. 3,608,998 and 4,076,018 and provides variable magnification elements near or in the eyepiece of the endoscope. Such an arrangement has the clear disadvantage that the resolution of the instrument is fixed once the front lens group (objective) is fixed. Thus, at the microscopic setting, the increased magnification is likely not to be accompanied by correspondingly increased resolution. A further difficulty with providing a variable magnification endoscope that allows true microscopic as well as telescopic examination arises from the great difficulty in correcting a lens for more than one set of conditions, unless a zoom lens having elements movable with respect to one another is used. While it is conceptually easy to visualize such a zoom lens at the distal end of the endoscope, such a solution is highly impractical due to the fact that zoom lens designs dictate a complex mechanical configuration and many lens elements. Since endoscope lens elements typically have a diameter in the neighborhood of two to three millimeters, a zoom lens is clearly impractical in the context of an endoscope.
U.S. Pat. No. 3,941,121 discloses an endoscope having an objective, the elements of which are fixed relative to one another, that is relatively movable with respect to a fiber optics transfer system. However, at the high magnification necessary for microscopy, the objective disclosed therein is incapable of producing the necessary image resolution. More specifically, it is impossible to obtain a sufficient numerical aperture at the level of correction necessary for performing pathological microscopy.
A further difficulty arises in the interaction between the objective and the transfer system. The use of a fiber optics transfer system tends to be accompanied by a loss in resolution. While this situation can be improved somewhat through the use of exotic techniques such as vibrating the input end of the fiber bundle or introducing aberrations which are subsequently corrected, maximum resolution is still likely to be achieved through the use of discrete lenses (alternating field and relay lenses). However, when discrete transfer optics are used, the problem of pupil coupling is aggravated. In particular, pupil coupling is critical in order to provide evenness of illumination, but where the objective moves relative to the transfer optics, the exit pupil location varies so much with large magnification changes that the image transfer to the eyepiece suffers unacceptable vignetting.
U.S. Pat. No. 4,111,529 discloses an endoscope objective for use in wide angle viewing, but the objective does not possess sufficient resolution to render it suitable for microscopic viewing. Moreover, the numerical aperture is insufficient due to the stop positioning disclosed.
Therefore, while variable magnification endoscopes are known, and microscopic endoscopes are known, there is presented the need for an endoscope that provides a continuous variation of magnification over a wide range while providing suitable corrections and pupil coupling under such widely varying conditions.